1. Field of the Invention
The invention pertains to an injection unit for an injection molding machine which injects plastic into a mold along an injection axis. The injection unit includes a set of stationary guide rods, and a first bridge extending between the guide rods. A plastification cylinder is carried by the first bridge, this plastification cylinder being for receiving, plastifying and discharging plastic to be molded. A plastification screw is disposed in the plastification cylinder. A hydraulic injection cylinder with an injection piston therein is coupled with the plastification screw for effecting an injection stroke of the plastification screw. There are a set of hydraulic drive cylinders coaxial with the quide rods to drive the injection unit. Each hydraulic drive cylinder includes a drive piston therein, the drive pistons being coupled with the guide rods. A second bridge extends between the guide rods and carries the plastification screw. A rotary drive motor is coupled with the plastification screw for rotatably driving the same.
2. Description of the Prior Art
In a known injection unit of this type (German Offenlegungsschrift No. 20 20 337 and corresponding U.S. Pat. No. 3,729,280), a rotary drive motor is positioned on a distinct bridge which is movable on stationary guide rods to prevent rotation. As a result, the distance between the front and rear guide rod support points must be great enough to enable forward and/or reverse strokes of the drive cylinder and to allow additionally for a decompression stroke while the injection unit is retracted from the injection mold. Under these conditions, repeated, cyclical bending of the guide rods during the injection cycles is unavoidable, because the injection unit is at a relatively great distance from the guide rod support points after retraction from the injection mod. It has been shown that precision guidance deteriorates when such injection units are subjected to intensive use over lengthy periods of time. This has a particularly disadvantageous effect when the nozzle forms part of the confines of the injection mold cavity.
In another known injection unit of the same general type (German Offenlegungsschrift No. 23 02 211), ribs are provided on the cylinders of the drive cylinders as support bearings to prevent rotation. These ribs form tracks for rollers on the rotary drive motor. When the decompression cylinders are activated, the rollers run up the tracks formed by the ribs, which thus receive a considerable load. On the other hand, the guide rods are neither designed not suited to rest on the machine pedestal due to their being made of several sections. Due to the unavoidable bending stress to which the guide rods and drive cylinders are subjected, an injection unit of this type is thus only possible in a smaller-sized embodiment